1. Field of the Invention
This invention relates broadly to a mixing apparatus and more particularly to such an apparatus of conical configuration with vertically mounted rotating screw elements which fully wipe the interior surfaces of the mixing apparatus and the surfaces of each other. The mixing apparatus is useful, for example, as a separator/finisher for producing high viscosity synthetic condensation polymers such as polyamides, polyesters, etc.
The term "mixing" used herein includes finishing high viscosity synthetic polymers, mixing two or more viscous liquids and blending solids and liquids together.
2. Description of the Prior Art
U.S. Pat. No. 3,717,330, issued Feb. 20, 1973 to B. M. Pinney, describes a mixing apparatus suitable for use as a separator/finisher in the production of synthetic condensation polymers such as polyamides and polyesters. The apparatus disclosed by Pinney includes a vessel having an interior surface throughout its length in the shape of at least two intersecting conical frustums with parallel and substantially vertical axes, the bases of the frustums being displaced upwards with respect to the apexes. Rotatably mounted within the vessel are at least two interengaging helical screw elements which upon co-rotation conform to the interior surface of the vessel such that the screw elements effect a complete wiping of the interior surface of the vessel, and the screw elements interengage each other uninterruptedly along their lengths such that each element effects a complete wiping of the adjacent element. The bottom portions of the screw elements form a pressure generating zone, and the top portions of the screw elements form a mixing zone having a hollow center described by the co-rotating screw elements. The screw elements frequently have multilobal cross-sections, for example, trilobal cross-sections, and in the mixing zone may be oriented so that the tip of the screw element or the "flank" of the screw element (arc of the multilobal cross-section instead of the tip of the multilobal cross-section) is adjacent to the vessel wall.
As used herein, the term "multilobal cross-section" means a cross-section bounded by a plurality of equal arcs of equal radii, the centers of the arcs being within the figure formed by joining adjacent intercepts of the arcs by straight lines and the terms "trilobal cross-section" and "pentalobal cross-section" means a cross-section bounded by three such equal arcs and a cross-section bounded by five such equal arcs, respectively.
When such an apparatus is used for the preparation of polymers, e.g., polyhexamethylene adipamide, which are particularly susceptible to thermal degradation, deposits of degraded polymer often referred to as "gel" may have a tendency to form in the upper part of the mixing zone, i.e., above the liquid (melt pool) level in the vessel. The tendency to form gel may be minimized by maintaining complete wetting and adequate shearing of all surfaces. Complete surface wetting can be maintained by requiring the helix angle (with a horizontal plane) of the screw elements above the liquid level in the vessel to be sufficiently high to allow the screw elements to convey polymer upwards from the melt pool and to distribute incoming polymer which enters through the top of the vessel around the periphery of the vessel. Adequate shearing between the screw elements and the vessel walls can be obtained by arranging the multilobal cross-section of each screw element above the liquid level such that a flank of the screw element is adjacent to the vessel wall, thus providing a greater duration of shearing between the screw elements and the vessel walls than is provided by arranging the multilobal cross-section of each screw element above the liquid level such that a tip of the screw element is adjacent to the vessel wall. However, regardless of the arrangement of the multilobal cross-sections of the screw elements with respect to the vessel walls, the screw element-to-screw element shear may be inadequate as the wiping action between the screw elements is a "tip wiping" type, i.e., a tip of one multilobal cross-section wipes a flank of the other multilobal cross-section. It was recognized that improved screw element-to-screw element shear could be achieved in the upper part of the mixing zone by utilizing screw elements of circular cross-section. However, screw elements of multilobal cross-section may be desirable in the pressure generating zone of the vessel.